Intestinal epithelial NAIP/NLRC4 function in infection

This research project aims to better understand how our immune system is able to detect infections in the gastrointestinal tract. The intestine is filled with useful microbial flora that is tolerated and not attacked by the immune system, despite the fact that disease causing and harmless gut bacteria (commensals) are generally very similar in their structures. We hypothesize, that not only immune cells but also epithelial cells lining the intestine can detect disease-causing bacteria using a detection system called the NAIP/NLRC4 inflammasome. This detection system allows cells to react to bacteria that have entered their inside, a behavior that usually does not occur in harmless commensal bacteria but is very common among infectious bacteria like Salmonella. To prove this hypothesis, we use new mouse models that can only detect bacterial structures inside their intestinal epithelial cells but not in their immune cells, or that lack the ability to detect certain bacterial structures inside all of their cells. We will study two models of infection: Salmonella, which can be the cause of severe food poisoning and typhoid fever, and Citrobacter rodentium, which is a mouse model of enteropathogenic/ enterohaemorrhagic E. coli (EPEC/EHEC), which cause bloody diarrhea, that can be deadly, especially in infants. These novel models will help us understand how the immune system detects gastrointestinal pathogens, which is crucial for our deeper insight into general immune processes sites as well as to develop therapies for infections with these pathogens.

In summary, we could describe a mechanism by which intestinal epithelial cells act as an efficient first defense against bacterial infections by employing a coordinated response. Inflammasomes are sensor molecules that get activated when bacteria, viruses or fungi or the toxins they produce enter a cell. They are important to help the immune system to distinguish between harmful, disease causing microorganisms and harmless ones such as our gut or skin bacteria. Such bacteria are often beneficial and usually live on the surface of tissues but do not try to enter them. The NAIP-NLRC4 inflammasomes specifically detect bacteria inside the bodys cells. It had already been shown, that in immune cells, recognition of bacteria by NAIP-NLRC4 leads to activation of a molecule called Caspase-1, which causes death of the infected cell and the release of molecules that stimulate the immune system. However, NAIP-NLRC4 also exists in epithelial (i.e. outer layer) cells of the intestine, which form a single layer that separates beneficial and potential harmful bacteria from the rest of the body. Its role in these cells was however less clear. In this study, we could show that when an intestinal epithelial cell is infected by bacteria such as Salmonella, the NAIP- NLRC4 inflammasome gets activated and leads to several results: (1) The infected cell dies and gets excluded from the tissue by the surrounding cells, which actively push it out and seal the resulting gap immediately. Furthermore, the dying cells release molecules, which (2) lead to the development of diarrhea and (3) the recruitment of immune cells. We speculate that this coordinated response leads to flushing out of the infected cells while any remaining bacteria can be attacked by the recruited immune cells. Another surprising finding we made was that NAIP-NLRC4 causes these effects not only by activating Caspase-1, a molecule that has been described as part of the inflammasome complex before, but can also activate Caspase-8. As many infectious bacteria are able to tamper with the immune system, we speculate that this is a backup mechanism in case bacteria have the ability to block Caspase-1. The results of this study help us to understand better how infections of the intestine lead to diarrhea and inflammation.